1. Field of The Invention
This invention generally relates to image processing and more particularly to an image reading system for setting values indicated by signals (hereunder referred to as density signals) representing gray levels (namely, densities) at a point of a high-light portion of an original image (hereunder referred to simply as a high-light point) and a point of a shadow portion of the original image (hereunder referred to simply as a shadow point) for predetermined values and for reading a manuscript in accordance with gray scale transformation characteristics determined according to the kind of the manuscript.
2. Description of The Related Art
In a prior art image reading system, a manuscript is first irradiated by light and next a quantity of light reflected or transmitted by the manuscript (hereunder referred to as a quantity of reading-light) is measured. Subsequently, the prior art reading system generates a signal (hereunder referred to as a density signal) representing densities of the manuscript from the result of the measurement of the quantity of reading-light and outputs the generated density signal. A method of generating a density signal, which is employed by the prior art reading system, will be described hereinbelow. In case of this method, in order to freely establish a corresponding relation between quantities of reading-light and values indicated by a density signal, a quantity of reading-light is first converted into a luminance signal by a photoelectric conversion portion and a signal amplification portion of the prior art reading system and thereafter a density signal is generated by referring to a look-up table (hereunder abbreviated as an LUT).
Generally, the following three kinds of methods are employed for establishing the LUT:
(1) A first method which uses a quantity of reading-light measured when reading a gray scale chart;
(2) A second method which uses quantities of reading-light measured correspondingly to at least three points (namely, a high-light point, a point of a half-tone portion (hereunder referred to as a half-tone point) and a shadow point) of a manuscript when reading the manuscript; and
(3) A third method which uses quantities of reading-light measured correspondingly to a high-light point and a shadow point of a manuscript when reading the manuscript. Further, in case where a picture is read in conformity of the range of gray levels of picture elements of the picture, the second or third method is employed. As a result of making a comparison between the second and third methods, it is found that the latter method can be performed by an operator more easily than the former method.
Additionally, the third method of establishing the LUT by using the quantities of reading-light corresponding to the high-light point and the shadow point on the manuscript has the following two types. A first type of the third method is comprised of the steps of preparing first LUT data corresponding to a positive manuscript and second LUT data corresponding to a negative manuscript, then writing the first or second LUT data to the LUT in accordance with the kind of the manuscript and subsequently regulating the signal amplification portion such that the quantities of reading-light corresponding to the high-light point and the shadow point are adjusted to operator's desired amounts.
The other type of the third method is comprised of the steps of first modifying a plurality of standard tone curve data stored in a memory such that a signal indicating data outputted from the LUT when signals representing quantities of reading-light corresponding to the high-light point and the shadow point are inputted thereto indicate desired values, then selecting the most appropriate data therefrom and subsequently writing the selected data to the LUT (see Japanese Patent Application Provisional Publication No. 60-37878 Official Gazette). In case of this type of the third method, the modification of the standard tone curve data g.sub.i (X) is performed by first obtaining parameters a and b of modified tone curve data (namely, a standard tone curve function of X-coordinates) g.sub.i ' (X) by using signals inputted to the LUT respectively representing the gray level X.sub.yh of the high-light point having X and Y coordinates (X.sub.ho, Y.sub.h) on the manuscript and that X.sub.ys of the shadow point having X and Y coordinates (X.sub.so, Y.sub.s) thereon. The parameters a and b are defined as follows. EQU a=(X.sub.so -X.sub.ho)/(X.sub.ys -X.sub.yh) EQU b=X.sub.ho -a*X.sub.yh
Then, the modified tone curve data are obtained by the following equation. EQU g.sub.i '(X)=g.sub.i (a*X+b) (1)
Such a modification is performed on all of the standard tone curve data. Thereafter, the most appropriate data are selected and is written to the LUT.
In case where a manuscript is read by the image reading device, it is necessary to establish a reference point on the manuscript and set the system such that the difference (O-O.sub.H) between the value O indicated by a density signal corresponding to a given point on the manuscript and that O.sub.H indicated by another density signal corresponding to the reference point thereon is obtained as a linear function of the difference (D-D.sub.H) between the density D at the given point and that D.sub.H at the reference point. Hereunder, this condition will be referred to simply as a linear condition. Incidentally, in case of the third method of setting the LUT by using the quantities of reading-light corresponding to a high-light point and a shadow point, the high-light point is employed as the reference point. Further, the linear condition can be expressed as follows: EQU (D-D.sub.H)=S.sub.D *(O-O.sub.H) (2)
where S.sub.D is a proportional factor (hereunder referred to as a density step). Furthermore, let Ds denote the density at the shadow point. In order to set the value indicated by the density signal corresponding to the shadow point as a desired value Os, the density step S.sub.D needs to be determined by the following equation: EQU S.sub.D =(Ds-D.sub.H)/(Os-O.sub.H) (3)
Additionally, when a fixed quantity of light is irradiated on a point of a positive manuscript, the relation between a quantity L of reading-light and a corresponding density D is expressed as follows: EQU D=-log ((L-L.sub.O)/C)
where C denotes a constant determined depending on the kind of the manuscript and the quantity of the irradiated light; and L.sub.O a quantity of reading-light when the fixed quantity of the light is not irradiated. Generally, the value indicated by a luminance signal I is represented by the following equation: EQU I=K*L
where K designates a constant; L a quantity of reading-light. Therefore, ##EQU1## where .alpha. is a given constant. Thus, as is understood from the equations (2) and (4), the linear condition expressed by the equation (2) holds if the relation between the value O indicated by the density signal indicating data outputted from the LUT and that I indicated by the luminance signal inputted to the LUT is given as follows: EQU O={log ((I.sub.H -I.sub.O)/.alpha.)-log ((I-I.sub.O)/.alpha.)/S.sub.D +O.sub.H ( 5).
Incidentally, data indicating the relation of a type as expressed by the equation (5) are referred to as gray scale transformation curve data (hereunder sometimes referred to as density transformation curve data). Furthermore, in order to make the value indicated by the density signal corresponding to the density condition equal to the desired value Os, the density step S.sub.D is determined from the equations (3) and (4) as follows: EQU S.sub.D ={log ((I.sub.H -I.sub.O)/.alpha.)-log ((Is-I.sub.O)/.alpha.)/(Os-O.sub.H) (6)
Thus, in order to satisfy the linear condition and meet the requirement that a manuscript should be read in such a manner that the values indicated by reading-signals respectively representing the density of the high-light point and that of the shadow point become predetermined levels, it is necessary to write data satisfying the equations (5) and (6) to the LUT before the reading of the positive manuscript is started. Therefore, in order to satisfy the above described requirement, the data as given by the equation (5) (namely, the data obtained by first dividing opposite logarithmic curve data by the density step S.sub.D and next adding a constant to the result of the division) should be written to the LUT.
Further, if the difference (Os-O.sub.H) between the desired values indicated by the reading-signals corresponding to the shadow point and the high-light point is considered to be a fixed value, the data of the LUT (hereunder sometimes referred to as the LUT data) are determined regardless of the difference (Ds-D.sub.H) between the densities at the high-light point and the shadow point in case of the prior art method of setting the LUT by writing specific data to the LUT according to the kind of the manuscript. However, as can be understood from the equation (3), the density step S.sub.D varies with the difference (Ds-D.sub.H) when the difference (Ds-D.sub.H) changes. Therefore, it can be concluded from the equation (5) that the data of the LUT for satisfying the linear condition should be changed and thus the above described requirement cannot be met by performing the prior art method.
Similarly, in case of the LUT setting method disclosed in the Japanese Patent Application Provisional Publication No. 60-37878 Official Gazette, the data of the LUT for satisfying the linear condition vary with the difference (Ds-D.sub.H) when the difference (Ds-D.sub.H) changes. Hereinafter, it is studied whether or not the system can generate data of the LUT satisfying the equation (5) such that the linear condition holds for the positive manuscript. First, let A, I and B denote the number of data inputted to the LUT, a natural number equal to or less than A and a given positive constant, respectively. Further, the following function g.sub.i (I) is employed as standard data: EQU g.sub.i (I)=(-1)*B*log (I/A)
In this case, the modified data g.sub.i (I) are obtained from the equation (1) as follows: ##EQU2## Thus, there is established the following relation between the values indicated by an output signal O and an input signal representing I of the LUT generated by using the modified data: EQU O=-B*{log ((I-b/a)/A)+log (a/A)} (7)
However, the constant B of the equation (7) is peculiar to the standard data g.sub.i (I) and is not necessarily equal to the density step S.sub.D which is determined according to the difference (Os-O.sub.H) between the desired values and the difference (Ds-D.sub.H) between the densities at the shadow point and the high-light point. Thus, the data of the generated LUT do not always satisfy the equations (5) and (6). Therefore, this prior art method cannot meet the above described requirement. Moreover, even if a plurality of standard data are used, the constant B will take a plurality of values. The above described requirement cannot be met by employing this prior art method for the same reason as in case of the third method of setting the LUT.
Further, when a fixed quantity of light is irradiated on a point of a negative manuscript, the relation between a quantity L of reading-light and a corresponding density D in a positive manuscript is established as the following function: EQU D=F((L-L.sub.O)/(L.sub.M -L.sub.O)) (8)
where L.sub.M denotes a quantity of reading-light when the quantity of the irradiated light is directly read or measured. As is understood from the equations (2) and (8), the linear condition is satisfied if the density O indicated by the density signal meets the following equation: EQU O-O.sub.H =S.sub.D *{F((L.sub.H -L.sub.O)/(L.sub.M -L.sub.O))-F((L-L.sub.O)/(L.sub.M -L.sub.O))} (9)
Furthermore, as is seen from the equations (3) and (8), the density step S.sub.D should be determined by the following equations for the purpose of making the density signal corresponding to the shadow point indicate a desired value Os: EQU S.sub.D =(Os-O.sub.H)/{F((Ls-L.sub.O)/(L.sub.M -L.sub.O))-F((L.sub.H -L.sub.O)/(L.sub.M -L.sub.O))} (10)
Hence, in cases of the conventional methods of setting the LUT, a negative manuscript cannot be read in a manner in which the above described requirement is met.
The present invention is created to solve the above described problems of the prior art system.
It is accordingly a first object of the present invention to provide an image reading system which can satisfy the linear condition and meet the requirement that a manuscript should be read in such a manner that the values indicated by reading-signals respectively representing the density of a high-light point and that of a shadow point become predetermined values.
Further, it is a second object of the present invention to provide an image reading system which can omit an operation of inputting density transformation data for frequently used manuscript from an external device and achieve the first object.